1. Field of the Invention
This invention relates to a liquid crystal display device, and more particularly to liquid crystal display devices applied for specific orienting films.
2. Description of the Prior Arts
Liquid crystal display devices are produced usually by preparing a pair of glass plates bearing transparent electrodes arranged in the pattern of picture elements, spacing the glass places face-to-face at a distance of 5 .mu.m to 100 .mu.m to obtain a sandwich cell, placing a liquid crystal composition into the cell and sealing the cell. The photoelectric characteristics of the liquid crystal cell is dependent largely on the orientation of the liquid crystal within the cell. Especially with liquid crystal display devices which operate under the action of an electric field (FEM liquid crystal display devices), there is required to show a uniform liquid crystal orientation. The orientation of the liquid crystal is usually controlled by a surface treatment of the cell, or by adding a very small amount of a chemical substance to the liquid crystal. As a mode of orientation, the liquid crystal molecules are aligned with their axes parallel to the substrate plane (homogeneous alignment) by rubbing the surface of the substrate with paper or fibers, or by forming an orienting film of oblique-deposited SiO.sub.2 on the substrate surface, while research has been conducted in recent years on the method of obtaining the desired orientation by preparing an orienting film form an organic high polymer and rubbing the film. This method appears very useful in view of the uniformity of the orientation available and the simplicity of the procedure.
Useful organic high polymers heretofore proposed for preparing orienting films for FEM liquid crystal display devices include polyimide (see Japanese Published Unexamined Patent Application No. 65960/1976), polyimideisoindoloquinazolinedione (see Japanese Published Unexamined Patent Application No. 30859/1979) and polyimidebenzoimidazopyrrolone (see Japanese Patent Application Disclosure No. 133359/1979). The orienting films prepared from such polymers followed by rubbing treatment induce a uniform orientation on liquid crystals, have high heat resistance and good insulating properties and will not readily permit erasion of the orientation even when heated to 100.degree. to 150.degree. C. for sealing. However, such conventional films of organic high polymers still fail to sufficiently inhibit the occurrence of reverse-tilt disclination (appearance of a domain of different viewing angles upon application of voltage). Additionally these films are not fully resistant to abrasion and are prone to wear over the surface when rubbed, so that when it is desired to produce an orienting film of a given thickness, the polymeric film to be subjected to the rubbing treatment must be prepared with a sufficiently larger thickness. In this case, nevertheless, difficulties are still encountered in obtaining a uniform orienting film with high reproducibility, consequently leading to the drawback that the resulting film involves variations in various properties.